In intra prediction encoding which performs prediction within a single frame, it is impossible to achieve compression efficiency as high as that achieved by inter prediction encoding which performs prediction between different frames. Therefore, an intra prediction encoding method for achieving a high degree of compression efficiency has been desired.
The intra prediction which performs prediction within a single frame in spatial dimensions has been employed in video coding standards from H.264/MPEG-4 AVC (e.g., see pp. 106-113 of Non-Patent Document 1).
The intra prediction is performed using a block unit, and in H.264/MPEG-4 AVC, three block sizes (4×4, 8×8, and 16×16) can be used for the luminance signal. For each block size, a plurality of prediction modes are provided for mode selection.
More specifically, for block sizes of 4×4 and 8×8, nine prediction modes are provided as shown in FIG. 5-3 on page 108 of Non-Patent Document 1 for the block size of 4×4, and in FIG. 11-6 on page 261 of Non-Patent Document 1 for the block size of 8×8, where eight prediction modes other than “prediction mode 2” have a prediction direction. Additionally, for a block size of 16×16, four prediction modes are provided as shown in FIG. 5-8 on page 111 of Non-Patent Document 1.
For the chrominance signal, only the block size of 16×16 can be used, and four prediction modes are provided as shown in FIG. 5-9 on page 112 of Non-Patent Document 1. For the prediction direction, the same definition as that applied to the 16×16 block size for the luminance signal is provided. However, the correspondence between the prediction mode number and the prediction direction is different between both signals.
The above items can also be explained in detail in lines 118 to 136 of Non-Patent Document 2.
For every block size and prediction mode, reference pixels used in the intra prediction always belong to a block adjacent to an encoding target block, and are closest to the encoding target block.
FIG. 18 shows a concrete example in which the encoding target block has a block size of 4×4 for the luminance signal, and a vertical prediction (prediction mode 0) is performed. In the following explanations, the luminance signal is always targeted unless a specific explanation is provided.
As shown in FIG. 18, with respect to the encoding target block, a pixel X in the upper-left block; pixels A, B, C, and D in the upper block; pixels E, F, G, and H in the upper-right block; and pixels I, J, K, and L in the left block can be used in the intra prediction.
Among the pixels, in the prediction mode 0 which is the vertical prediction, the value of pixel A is assigned to pixels a, e, i, and m; the value of pixel B is assigned to pixels b, f, j and n; the value of pixel C is assigned to pixels c, g, k, and o; and the value of pixel D is assigned to pixels d, h, l, and p.
In the intra prediction, there is no block to be referred to in accordance with the block position. In such a case, a value of “128” or the value of an adjacent pixel is set for performing the prediction.
For example, in a block positioned at the uppermost line in the frame, nine pixels from X to H can never be referred to, and thus the value “128” is used. In addition, if there are upper-left and upper blocks but no upper-right block is present, the value of pixel D is assigned to each of pixels E, F, G, and H.    Non-Patent Document 1: Sakae Okubo, Shinya Kadono, Yoshihiro Kikuchi, and Teruhiko Suzuki, Revised edition of “H.264/AVC TEXTBOOK”, pp. 106-112, 259-262, 2006    Non-Patent Document 2: ITU-T Study Group16-Question 6/16: “Draft new Corrigendum 1 to H.264 “Advanced video coding for generic audiovisual services””, pp. 118-136, 2005